There are automobiles with antennas for radiating radar waves and receiving reflected waves, each of which is mounted at the front nose or in the vicinity of the rear gate. However, these parts of the automobiles are the first to be deformed or damaged in cases of collisions with other vehicles or objects, even if the collisions are minor ones, and a radar mounted on such a part is also highly likely to be damaged. The radar is a device that is necessary to ensure the safety of automobiles, and it is not desirable for the radar to lose its functionality due to minor collisions. This is all the more so if automatic driving is put into practical use.
Such undesirable situations are less likely to occur if the radar device is mounted in the interior of a vehicle, but in that case, the radar device needs to transmit and receive radar waves through the windshield including glass. In this case, the reflection and absorption of the waves by the glass are unavoidable, and the radar will have limited detection capabilities.
European Patent No. 888646 discloses that, when a communication antenna is installed in the interior of a vehicle, an intermediate dielectric member is inserted between glass and the radiating surface of the antenna in order to suppress the reflection of a radio wave by the glass. According to European Patent No. 888646, the electrically effective distance between the glass and the antenna is adjusted to several times the half-wavelength of the wave.
When a radio wave in the millimeter waveband is used as a transmission wave, strong reflection will occur on the surface of the windshield including glass. Even in the case where the intermediate dielectric member is arranged between the glass and the radiating surface of the antenna as in European Patent No. 888646, strong reflection will occur on the surface of the dielectric member itself. Additionally, since the windshield is ordinarily inclined with respect to the radiating surface of the antenna, the distance between the glass and the antenna cannot be adjusted to a constant value, such as several times the half-wavelength of the radio wave. Thus, there is demand for a new technique for reducing loss of a transmission wave passing through the windshield.